Metric variation in the postcranial skeleton of ostriches,Struthio(Aves: Palaeognathae), with new data on extinct subspecies

Abstract

As a result of numerous fossil and subfossil finds of ostriches, there is great demand for a comprehensive osteometric dataset for the living species and subspecies of the genus Struthio. We meet this demand by providing a set of > 100 measurements for a sample of 18 sexed skeletons, including all living and recently extinct species and subspecies of ostriches. We provide the first mensural data for two extinct subspecies, the hitherto questioned Struthio camelus spatzi from north-western Africa and the Arabian ostrich, Struthio camelus syriacus. The unique skeletal proportions of S. c. spatzi, with a relatively short wing, broad pelvis, short tarsometatarsus and big third toe, confirm the validity of this taxon and suggest an increased stability at the expense of cursoriality, which might have contributed to its extermination by humans. Our biometric analysis of the entire sample suggests a subtle sexual dimorphism in the ostrich skeleton, with females having more robust limb bones (especially wider and/or deeper at the ends) despite being on average smaller than males. If confirmed by further research, this size-independent dimorphism might reflect the independent regulation of the longitudinal and transverse dimensions of bones as revealed by several independent studies of morphological integration (covariance among morphological traits) in the avian skeleton.

Femora from an exceptionally large population of coeval ornithomimosaurs yield evidence of sexual dimorphism in extinct theropod dinosaurs

“…To do so, they coupled morphometrics and multivariate statistics with the observation of medullary bone, a sex-specific tissue present in bones of egg-laying female in archosaurians (Dacke et al, 1993;Schweitzer et al, 2005Schweitzer et al, , 2007Canoville et al, 2019). The same kind of sexual dimorphism was observed in modern birds like California gulls (Larus californicus) (Schnell et al, 1985) and in the two extant species of ostriches (Struthio c. camelus, S. c. molybdophanes), but with reversed proportions between males and females (Elzanowski and Louchart, 2022). Furthermore, (Duggan et al, 2015) demonstrated that young male domestic ducks (Anas platyrhynchos) had more laterally curved femora than females, and that this sexually dimorphic feature disappeared along ontogeny.…”

“…However, Elzanowski & Louchart (2022) demonstrated that female ostriches had more robust limb bones but smaller average body size than males. This decoupling between size and shape dimorphism is concordant with our results and emphasizes that sexual dimorphism is not necessarily reflected by body size nor allometry between limb segments.…”

“…There are many examples of observations and/or inferences of SSD and allometric relationships in extant and extinct dinosaurs (Larson, 1994; Bunce et al, 2003; Clarke, 2004; Székely et al, 2007; Remeš and Székely, 2010; Olson and Turvey, 2013; Handley et al, 2016; Manin et al, 2016; Fajemilehin, 2017). However, Elzanowski & Louchart (2022) demonstrated that female ostriches had more robust limb bones but smaller average body size than males. This decoupling between size and shape dimorphism is concordant with our results and emphasizes that sexual dimorphism is not necessarily reflected by body size nor allometry between limb segments.…”

“…The closest extant relatives of non-avian dinosaurs are known to display sexual dimorphism with more or less visibility: birds display variation in their plumage and skeleton (Schnell et al, 1985;Owens and Hartley, 1998;Dunn et al, 2001;Székely et al, 2007;Clarke, 2013;Duggan et al, 2015;Manin et al, 2016;Hone and Mallon, 2017;Elzanowski and Louchart, 2022), whereas the variation is restricted to skeleton in crocodilians (Fitch, 1981;Farlow et al, 2005;Cox et al, 2007;Prieto-Marquez et al, 2007;Bonnan et al, 2008;Hone and Mallon, 2017;Hone et al, 2020). The extant phylogenetic bracket (EPB) of non-avian dinosaurs (Witmer and Thomason, 1995) thus implies they were sexually dimorphic too (Hone and Mallon, 2017;Hone et al, 2020).…”

“…S2 & S3). However, sexual dimorphism was observed in the proximal ends of femora in extant ostriches (Charuta et al, 2007; Elzanowski and Louchart, 2022) and California gulls (Schnell et a l., 1985). In addition, the anteroposterior width of the femoral shaft was demonstrated to vary between sexes among savannah sparrows ( Passerculus sandwichensis ; Rising, 1987) and three species of steamer-ducks ( Tachyeres pteneres , T. leucocephalus , T. patachonicus , (Livezey and Humphrey, 1984).…”